OCT interferometric methods deliver light onto a sample of interest, such as the wall of a lumen of a blood vessel, and collect a portion of the light returned from the sample. Due to the size and complexity of many light sources and light analysis devices, the sources and light detectors are typically located remotely from the sample area of interest. One method of optically analyzing internal parts is to guide light from a remote light source onto the sample using a thin optical fiber that is minimally disruptive to the normal function of the sample. This minimal disruption occurs because of the diminutive cross-section of the optical fiber.
There are many miniature optical systems known in the art that can be used for analysis of internal lumen structures. Each optical system can be conceptually divided into a beam delivery and focusing means, and a beam directing means. Light is passed from an external light source to the internal lumen through one or more optical illumination fibers, which may be single mode or multimode in nature. The illumination fiber is in communication with the miniature optical system, which focuses and directs the beam into the luminal wall.
Light is reflected by the lumen wall and transmitted to an analysis apparatus outside the body, generally using the same fiber as transmitted the incident light. The analysis apparatus is typically interferometric and the resulting interferometric patterns are detected and transformed into an image by a computer. In use, the fiber spins within the lumen of the vessel, thereby sweeping the wall with the light and collecting the reflected light. Each revolution of the fiber therefore produces a scanned cross-sectional image of the vessel. As the fiber is retracted or pulled out of the vessel, a cylindrical image of a portion of the vessel lumen is obtained.
An IVUS probe is similar to an OCT probe; however, the IVUS probe uses ultrasound rather than light. Ultrasonic pulses are produced by an IVUS transducer at the probe tip and the sound reflected by the walls of the lumen is received by the transducer and converted into electrical signals which are then analyzed by a computer. As with OCT imaging, the IVUS probe spins in the lumen of the vessel and this results in a reflection pattern from the walls of the vessel that is also analyzable by computer to produce a cross-sectional image. Again, as the IVUS probe is withdrawn from the vessel, a cylindrical image of a portion of the vessel lumen is obtained.
Now that combination OCT and IVUS probes have become available, it is possible to acquire both images in one procedure. Obtaining both of the IVUS and OCT images of the vessel has benefits, because of the inherent advantages and limitations of each imaging modality. OCT light tends to be scattered by particles, such as blood cells passing through the lumen of the vessel. This scattering degrades the resulting OCT image of the vessel wall. However, OCT light penetrates into the wall of the vessel thereby being able not only to image the wall of the vessel but also the intima of the vessel. IVUS ultrasonic pulses are not as affected by particles in the lumen. Therefore, IVUS and OCT images can complement one another.
Because the two images, whether taken with the same probe or different probes, have different resolution, the alignment of the images to form a usable composite image is difficult. In addition, because of other limitations that will be discussed below, accurate determination of actual distances in the images is difficult. What is needed is a way to process the OCT and IVUS images so that both images depict the same region of the vessel, at the same magnification and orientation, and with a mechanism to check the calibration of each imaging modality.
The present invention addresses this need.